How to fix sagging composite deck railing

It appears that the rim joist to which the railing is attached was nailed/screwed into the end grain of the floor joists. This is an exceptionally weak connection and why toe nailing is standard when fastening perpendicular framing. If this is the case, new screws or bolts won't really change anything.

My recommendation is use framing connectors such as those manufactured by Strong-Tie and USP. These come in a variety of configurations such as angles and plates. They are also manufactured in a variety of sizes, gauges, and to receive varying numbers of fasteners. They are reasonably easy to install for even a casual carpenter and reduce the likelihood of splitting the wood members and struggling with bent nails. This comes at a higher material cost, but not of the sort that will break a reasonable budget.

Without seeing a picture it is hard to say.

My experience with wooden furniture is that when you put it outdoors bad things happen, especially if screws and nails are used to hold it together. It is pretty rare to see a deck older than 10 years that does not look like a potential termite habitat.

In general, putting corner braces under the railing would certainly provide a lot more support than a screw through the top. Unless you bolt the corner braces you will still have the screw deterioration problem. This can be mitigated by using brass or stainless steel screws. Bolts, of course, tend to have that Frankenstein look, but at least they hold the wood heap together.

My guess is that one to two years from now we will start getting questions about decking deterioration from you.